Solution rubbers containing hydroxyl groups

ABSTRACT

The invention relates to rubber blends containing said at least one solution-polymerized rubber containing hydroxyl groups, synthesized from diolefins and vinylaromatic monomers, wherein said at least one solution-polymerized rubber containing hydroxyl groups contain in the region of 0.1 to 5 wt. % of bonded hydroxyl groups, to a process for their preparation and to their use for the production of all kinds of moldings.

This application is a divisional application of U.S. Ser. No. 09/350,261which was filed on Jul. 8, 1999.

FIELD OF THE INVENTION

The present invention relates to rubber blends containingsolution-polymerized rubbers with a hydroxyl group, especially primary,content of 0.1 to 5 wt. %, and their mixtures with fillers, optionallyother rubbers and rubber aids, and to vulcanized products manufacturedtherefrom. The rubber blends according to the present invention aresuitable for the production of highly reinforced, abrasion-resistantmoldings, especially for the manufacture of tires which have aparticularly high wet grip.

BACKGROUND OF THE INVENTION

Compared with corresponding emulsion rubbers, anionically polymerizedsolution rubbers containing double bonds, such as solution polybutadieneand solution styrene/butadiene rubbers, have advantages in themanufacture of tire treads with a low rolling resistance. The advantageslie inter alia in the ability to control the vinyl content and theassociated glass transition temperature and the molecular branching.This gives rise in practical use to particular advantages in therelationship between the wet grip and the rolling resistance of thetire. Thus, U.S. Pat. No. 5,227,425 describes the manufacture of tiretreads from a solution SBR and silica. Numerous methods of end groupmodification have been developed to improve the properties further,e.g., with dimethylaminopropylacrylamide as described in EP-A 334,042 orwith silyl ethers as described in EP-A 447,066. Because of the highmolecular weight of the rubbers, however, the proportion by weight ofthe end group is small and cannot, therefore, greatly influence theinteraction between filler and rubber molecule. One object of thepresent invention is to prepare solution SBRs with a markedly highercontent of effective groups for interaction with the filler.

Solution polybutadiene rubbers containing hydroxyl groups are alsodescribed in DE-OS 2,653,144. However, because their strength is toolow, these rubbers are not suitable as the main component in tiretreads.

EP-A 464,478 describes a process for the hydroxylation of rubbers, butthis involves the introduction of secondary hydroxyl groups, which arefar less effective than the primary hydroxyl groups of the presentinvention.

Emulsion and solution rubbers containing hydroxyl groups are alsodescribed in EP 806,452 A1, the hydroxyl contents described in this casefor solution rubbers being in an appreciably lower range (0.009 to0.061%) as a consequence of the process. The present invention showsthat these contents have no significant effect on the wet grip.

SUMMARY OF THE INVENTION

It has now been found that rubber blends and vulcanized rubber productswith surprisingly improved dynamic damping properties in the temperaturerange relevant to wet grip and in the temperature range relevant torolling resistance, as well as improved abrasion behavior, can beprepared from solution vinylaromatic/diolefin rubbers containinghydroxyl groups with a bonded hydroxyl group, especially, a primaryhydroxyl group, content of 0.1 to 5 wt. % and a 1,2-vinyl content of 5to 60 wt. %. Other surprising advantages were obtained when the rubberblend was prepared not in a kneader, as is customary, but by mixing asolution of rubber containing hydroxyl groups and oxide or silicatefiller in an organic solvent, and then removing the solvent with steam,because in that case, the filler is completely precipitated with therubber and does not remain in the effluent, as would be the case whenusing unmodified rubber.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the dynamic damping curves against temperature for Examples5 and 8.

DETAILED DESCRIPTION OF THE INVENTION

Therefore, the present invention provides rubber blends containing oneor more solution-polymerized rubbers containing hydroxyl groups,synthesized from diolefins and vinylaromatic monomers, wherein thesolution-polymerized rubber(s) containing hydroxyl groups contain in theregion of 0.1 to 5 wt. % of bonded hydroxyl groups, together withfillers and optionally other rubbers and rubber aids, and the use ofsaid rubber blends for the manufacture of vulcanized rubber products,especially silica-filled tire treads with particularly high abrasionresistance, particularly, high wet grip and low rolling resistance.

The solution-polymerized vinylaromatic/diolefin rubbers advantageouslyhave average molecular weights (number-average) of 50,000 to 2,000,000and glass transition temperatures of −50° to +20° C.

The bonded hydroxyl groups are primary, secondary or tertiary,preferably primary or secondary.

Suitable vinylaromatic monomers are styrene, o-, m- and p-methylstyrene,p-tert-butylstyrene, α-methylstyrene, vinylnaphthalene, divinylbenzene,trivinylbenzene and divinylnaphthalene. Styrene is particularlypreferred.

Suitable diolefins are especially 1,3-butadiene, isoprene,1,3-pentadiene, 2,3-dimethylbutadiene, 1-phenyl-1,3-butadiene and1,3-hexadiene. 1,3-Butadiene and isoprene are particularly preferred.

The preparation of the rubbers according to the present invention forthe rubber blends is effected by anionic solution polymerization, i.e.,by means of a catalyst based on an alkali metal, e.g., n-butyllithium,in a hydrocarbon as solvent. It is additionally possible to use theknown randomizers and control agents for the microstructure of thepolymer. Anionic solution polymerizations of this type are known and aredescribed e.g., in I. Franta, Elastomers and Rubber CompoundingMaterials, Elsevier 1989, pages 73-74 and 92-94, and in Houben-Weyl,Methoden der Organischen Chemie (Methods of Organic Chemistry), ThiemeVerlag, Stuttgart, 1987, volume E 20, pages 114 to 134. The primaryhydroxyl groups are introduced in a subsequent reaction on the finishedpolymer. Methods of introducing the primary hydroxyl groups are, e.g.,the addition of mercaptans containing primary hydroxyl groups, anaddition reaction with formaldehyde, reaction with carbon monoxidefollowed by hydrogenation, and hydroboration of the vinyl groups of theL-SBRs followed by oxidative hydrolysis of the borane compound.

Examples of suitable alkali metal polymerization catalysts in terms ofthe present invention are lithium, sodium, potassium, rubidium, cesiummetal and their hydrocarbon compounds and complex compounds with polarorganic compounds.

Lithium and sodium hydrocarbon compounds having 2 to 20 carbon atoms areparticularly preferred, examples being ethyllithium, n-propyllithium,i-propyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-decyllithium, phenyllithium, 2-naphthyllithium, 2-butylphenyllithium,cyclohexyllithium, 4-cyclopentyllithium, 1,4-dilithio-2-butene, sodiumnaphthalene, sodium biphenyl, potassium/tetrahydrofuran complex,potassium/diethoxyethane complex and sodium/tetramethyl-ethylenediaminecomplex. The catalysts can be used independently of one another or in amixture.

Preferred amounts of catalysts are between 0.2 and 15 mmol/100 gpolymer.

The anionic solution polymerization is carried out in a hydrocarbon orin another solvent which does not adversely affect the catalyst, forexample tetrahydrofuran, tetrahydropyran or 1,4-dioxane. Examples ofhydrocarbons which are suitable as solvents are aliphatic,cycloaliphatic or aromatic hydrocarbons having 2 to 12 carbon atoms.Preferred solvents are propane, butane, pentane, hexane, cyclohexane,propene, butene, 1-pentene, 2-pentene, 1-hexenes, 2-hexene, benzene,toluene and xylene. The solvents can be used on their own or as amixture.

The hydroxyl groups are preferably introduced by means of an additionreaction with hydroxymercaptans of general formula (1) and/ormercaptocarboxylic acid esters containing hydroxyl groups of generalformula (2). The reaction is preferably carried out in solution,optionally, in the presence of free radical initiators.

HS—R¹—OH  (1)

HS—(CHR²)_(n)—(CO₂—R³—OH)_(m)  (2)

wherein

R¹ is a linear, branched or cyclic C₁-C₃₆ alkyl group which canoptionally be substituted by up to 6 further hydroxyl groups or can beinterrupted by nitrogen, oxygen or sulfur atoms,

R² is hydrogen or a C₁-C₆ alkyl group,

R³ is a linear, branched or cyclic C₂-C₃₆ alkyl group which canoptionally be substituted by up to 6 further hydroxyl groups or can beinterrupted by nitrogen, oxygen or sulfur atoms,

OH is a hydroxyl group, preferably primary

n is an integer from 1 to 5 and

m is an integer from 1 to 2.

Preferred hydroxymercaptans are mercaptoethanol, 1-mercapto-3-propanol,1 -mercapto-4-butanol, α-mercapto-ω-hydroxyoligoethylene oxides, e.g.,α-mercapto-ω-hydroxyoctaethylene glycol, or the corresponding ethyleneoxide/propylene oxide copolyethers. Mercaptoethanol andα-mercapto-ω-hydroxyoligoethylene oxides are particularly preferred.

Preferred mercaptocarboxylic acid esters containing hydroxyl groups areesters of mercaptoacetic acid, mercaptopropionic acid andmercaptobutyric acid with ethylene glycol, propylene glycol, butyleneglycol, diethylene glycol, triethylene glycol, tetraethylene glycol,octaethylene glycol, dipropylene glycol, tripropylene glycol,tetrapropylene glycol and N-methyldiethanolamine. The correspondingesters of mercaptoacetic acid and 3-mercaptopropionic acid areparticularly preferred.

Suitable free radical initiators for the addition of thehydroxymercaptans onto the solution rubbers are, e.g., azo initiatorssuch as azobisisobutyronitrile and azobiscyclohexanenitrile, andperoxides such as dilauroyl peroxide, benzpinacol silyl ether, orphotoinitiators in the presence of UV or visible light. Diacylperoxides, especially dilauroyl peroxide, didecanoyl peroxide,di(3,3,5-trimethylhexanol) peroxide, disuccinoyl peroxide and dibenzoylperoxide, are particularly preferred.

Preferred amounts of free radical initiators are 0.5 to 10 wt. %, basedon hydroxymercaptan.

The Mooney viscosity ML 1+4 of the copolymers is between 10 and 200,preferably 30 to 150, measured at 100° C.

The content of copolymerized 1,2-butadiene units (“vinyl content”) isbetween 5 and 60 wt. %, preferably 10 to 50 wt. %.

The content of copolymerized vinylaromatic compound is between 5 and 40wt. %, preferably 10 to 30 wt. %.

The content of hydroxyl groups is between 0.1 and 5 wt. %, preferably inthe range 0.1 to 3 wt. %, particularly preferably in the range 0.3 to 2wt. % and very particularly preferably in the range 0.5 to 2 wt. %,based on rubber.

The hydroxyl group content can be determined by known methods, e.g., byspectroscopy, titrimetry or elemental analysis or by determination ofthe so-called hydroxyl number (OH number), i.e., by reaction withreagents which eliminate titratable acids in contact with OH groups, cf.DIN 53240.

The solution-polymerized rubbers containing hydroxyl groups can be usedon their own, extended with aromatic or aliphatic oils or blended withother rubbers. As well as natural rubber, synthetic rubbers are alsosuitable as additional rubbers for the manufacture of vulcanized rubberproducts. Examples of preferred synthetic rubbers are described by W.Hofmann, Kautschuktechnologie (Rubber Technology), Gentner Verlag,Stuttgart, 1980, and I. Franta, Elastomers and Rubber CompoundingMaterials, Elsevier, Amsterdam, 1989. They include inter alia:

BR—polybutadiene

ABR—butadiene/C₁-C₄ alkyl acrylate copolymers

CR—polychloroprene

IR—polyisoprene

SBR—styrene/butadiene copolymers with styrene contents of 1 to 60,preferably 20 to 50 wt. %

IIR—isobutylene/isoprene copolymers

NBR—butadiene/acrylonitrile copolymers with acrylonitrile contents of 5to 60, preferably 10 to 40 wt. %

HNBR—partially hydrogenated or completely hydrogenated NBR

EPDM—ethylene/propylene/diene copolymers

and blends of these rubbers. The following are of particular interestfor the manufacture of motor vehicle tires with the aid ofsurface-modified fillers: natural rubber, emulsion SBRs and solutionSBRs with a glass transition temperature above −50° C., which canoptionally be modified with silyl ethers or other functional groups,such as those described, e.g., in EP-A 447,066, polybutadiene rubberwith a high 1,4-cis content (>90%), which is prepared with catalystsbased on Ni, Co, Ti or Nd, and polybutadiene rubber with a vinyl contentof 0 to 75%, as well as blends thereof.

The rubber blends according to the invention contain 5 to 300 parts byweight of an active or inactive filler, e.g.,

highly dispersed silicas, prepared, e.g., by the precipitation ofsilicate solutions or the flame hydrolysis of silicon halides, withspecific surface areas of 5 to 1000, preferably 20 to 400 m²/g (BETspecific surface area), and with primary particle sizes of 10 to 400 nm;the silicas can optionally also be present as mixed oxides with othermetal oxides, such as those of Al, Mg, Ca, Ba, Zn, Zr and Ti;

synthetic silicates, such as aluminum silicate and alkaline earth metalsilicate like magnesium silicate or calcium silicate, with BET specificsurface areas of 20 to 400 m²/g and primary particle diameters of 10 to400 nm;

natural silicates, such as kaolin and other naturally occurring silica;

glass fibers and glass fiber products (matting, extrudates) or glassmicrospheres;

metal oxides, such as zinc oxide, calcium oxide, magnesium oxide andaluminum oxide;

metal carbonates, such as magnesium carbonate, calcium carbonate andzinc carbonate;

metal hydroxides, e.g., aluminum hydroxide and magnesium hydroxide;

carbon blacks; the carbon blacks to be used here are prepared by thelamp black, furnace black or gas black process and have BET specificsurface areas of 20 to 200 m²/g, e.g., SAF, ISAF, HAF, FEF or GPF carbonblacks;

rubber gels, especially those based on polybutadiene, butadiene/styrenecopolymers, butadiene/acrylonitrile copolymers and polychloroprene.

Highly dispersed silicas and carbon blacks are particularly preferred.

The above-mentioned fillers can be used independently of one another orin a mixture. In one particularly preferred embodiment, the fillerspresent in the rubber blends consist of a mixture of light fillers, suchas highly dispersed silicas and carbon blacks, the mixing ratio of lightfillers to carbon blacks being 0.05 to 20, preferably 0.1 to 10.

The fillers are preferably added as solids or as a slurry in water or asolvent to a solution of the solution-polymerized rubber(s) containinghydroxyl groups. The rubber solution can be prepared beforehand, but thesolution originating from the polymerization is preferably useddirectly. The solvent is then removed by heating or, preferably, withthe aid of steam. The conditions of this stripping process can easily bedetermined by preliminary experiments.

As a further preference, the fillers are added to the solid rubbercontaining hydroxyl groups, or to a blend of rubbers, and incorporatedin known manner, e.g., with a kneader.

For the preparation of the rubber blends, according to this invention,the content of hydroxyl groups in an amount from 0.1 to 5 wt. % is thecrucial key-feature, the nature of the hydroxyl groups (primary,secondary or tertiary) or the nature of the rubber is a minor issue.

The rubber blends, according to the present invention, optionally,contain crosslinking agents as well. Crosslinking agents which can beused are sulfur or peroxides, sulfur being particularly preferred. Therubber blends according to the present invention can contain furtherauxiliary products for rubbers, such as reaction accelerators,anti-aging agents, heat stabilizers, light stabilizers, ozonestabilizers, processing aids, plasticizers, tackifiers, blowing agents,dyestuffs, pigments, waxes, extenders, organic acids, inhibitors, metaloxides, and activators, such as, triethanolamine, polyethylene glycol,hexanetriol, etc., which are known to the rubber industry.

It is particularly advantageous to use additional filler activators inthe preferred rubber blends with highly active precipitated silicas.Preferred filler activators are sulfur-containing silyl ethers,especially bis(trialkoxysilylalkyl) polysulfides, as described in DE2,141,159 and DE-AS 2,255,577, the oligomeric and/or polymericsulfur-containing silyl ethers of DE-OS 4,435,311 and EP-A 670,347,mercaptoalkyltrialkoxysilanes, especially mercaptopropyltriethoxysilane,and thiocyanatoalkylsilyl ethers, as described, e.g., in DE-OS19,544,469.

The rubber aids are used in conventional amounts, which depend interalia on the intended use. Conventional amounts are, e.g., from 0.1 to 50wt. %, based on rubber.

The rubber blends according to the invention are outstandingly suitablefor the production of all kinds of moldings.

Non-limiting examples of these moldings are O-rings, profiles, seals,membranes, tires, tire treads, damping elements and hosing. Tires andtire treads are particularly preferred.

The invention is further illustrated but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLE 1

25 g of 1-mercapto-2-ethanol and 1 g of dilauroyl peroxide are added at70° C. to a solution of 500 g of Buna VSL 5025-0 solution SBR (Bayer AG,bonded styrene content 25 wt. %, 1,2-bonded butadiene content 50 wt. %)in 4 l of cyclohexane. The mixture was subsequently stirred for 16 hoursat 70° C. 2.5 g of Vulkanox BKF antioxidant (Bayer AG) were then addedand the solvent was distilled off with steam. After drying at 70° C.under vacuum, 525 g of a colorless rubber with a glass transitiontemperature (DSC) of −11° C., an OH number of 34 and an OH content of1.04 wt. % were obtained.

Example 2 The procedure of Example 1 was repeated using the followingamounts: Buna VSL Free radical OH content 5025-0 solution Hydroxy-initiator and Glass transition of the end Example SBR mercaptan reactiontime temperature product 2 500 g 12.5 g of 1- 0.2 g of −16° C. 0.53 wt.% mercapto-2- azobiscyclo- ethanol hexanenitrile 16 hours/80° C.

EXAMPLES 3 AND 4 (COMPARATIVE EXAMPLES)

As comparative examples, a solution SBR with a low hydroxyl groupcontent and a solution SBR containing secondary hydroxyl groups wereprepared with the aid of 1-mercapto-2-hydroxydodecane, known from EP464,478. The same starting rubber was used. The procedure of Example 1was repeated using the following amounts:

ML 1 + 4 Buna VSL Free radical OH content Comparative 5025-0 solutionHydroxy- initiator and of the end Example SBR mercaptan reaction timeproduct 3 500 g 1.6 g of 1- 0.2 g of 52 0.07 wt. % mercapto-2-ethanolazobiscyclo- hexanenitrile 16 hours/80° C. 4 500 g 73.5 g of 1- 1 g ofdilauroyl 55 1.0 wt. % (according to mercapto-2- peroxide EP 464,478)hydroxydodecane 3 hours/80° C.

EXAMPLES 5-9

The following rubber blends (except for the sulfur and accelerator) wereprepared at 140-150° C. (ejection temperature) in a 1.5 l kneader.Mixing time: 5 minutes. The sulfur and accelerator were incorporated atthe end at approx. 50°-70° C. on a roller.

Comparative Comparative Comparative Constituent Example 5 Example 6Example 7 Example 8 Example 9 Buna VSL 5025-0 70 0 0 0 0 (Bayer AG)Rubber according 0 0 0 70 0 to Example 1 Rubber according 0 0 0 0 70 toExample 2 Rubber according 0 70 0 0 0 to Example 3 (Comparative) Rubberaccording 0 0 70 0 0 to Example 4 (Comparative) Buna CB 25 30 30 30 3030 Vulkasil S 70 70 70 70 70 (Bayer AG) Silane Si 69 6 6 6 6 6 (Degussa)Carbon black 10 10 10 10 10 N121 (Degussa) Zinc oxide RS 2.5 2.5 2.5 2.52.5 (Bayer) Stearic acid 1 1 1 1 1 Vulkanox 4020 1 1 1 1 1 (Bayer)Sulfur 1.5 1.5 1.5 1.5 1.5 Vulkacit CZ 1.8 1.8 1.8 1.8 1.8 Vulkacit D 22 2 2 2

The rubber blends were then vulcanized for 20 minutes at 170° C. Thevulcanized products had the following properties: Property of vulcanizedComparative Comparative Comparative product Example 5 Example 6 Example7 Example 8 Example 9 Tensile 14.3 15.9 16.3 17.0 15.5 strength (MPa)⁽¹⁾Elongation at 350 365 380 340 330 break (%)⁽¹⁾ Tensile stress 3.1 3.2 33.4 3.4 at 100% elongation (MPa)⁽¹⁾ Tensile stress 11.9 12.2 11.9 14.613.6 at 300% elongation (%)⁽¹⁾ Shore A 71 70 69 73 69 hardness (23°C.)⁽²⁾ Shore A 68 65 65 71 65 hardness (70° C.)⁽²⁾ Rebound 32 30 28 2528 resilience at 23° C. (%)⁽³⁾ Rebound 50 50 49 51 53 resilience at 70°C. (%)⁽³⁾ Difference 18 20 21 26 25 between rebound resiliences at 23°and 70° C. Abrasion (DIN 93 n.d. 87 n.d. 53516) (ccm) ⁽¹⁾determined bytensile testing according to DIN 52504 with standard bar 2 ⁽²⁾determinedaccording to DIN 53505 ⁽³⁾determined according to DIN 53512

The experimental results show that the mechanical properties and theabrasion behavior have been improved compared with the unmodifiedrubber; also, the rebound resilience measured at room temperature wasmarkedly lower, which is shown by experience to be associated with aconsiderable improvement in the wet grip. The difference between therebound resiliences at room temperature and 70° C. is markedly greaterin the case of the rubber blends according to the invention, so therelationship between the wet grip and the rolling resistance of the tireis also appreciably more favorable. The solution SBRs modified with fewhydroxyl groups (rubber according to Example 3) and solution SBRsmodified with secondary hydroxyl groups (rubber according to Example 4),as in the known state of the art, lie in the difference between therebound resiliences at room temperature and 70° C. in the region of theunmodified rubbers, so the relationship between the wet grip and therolling resistance of the tire is not significantly improved here.

The dynamic damping behavior of the vulcanized rubber products in thetemperature range from approx. −10 to +80° C. is of particularimportance for evaluating the rolling resistance and wet grip behavior.The damping is required to be as high as possible at −10° to +10° C. andas low as possible in the temperature range from 50° to 80° C. FIG. 1below shows the dynamic damping curves against temperature, measured bymeans of a Roehlig instrument (DIN 53513). It is clearly seen that thevulcanized product according to the present invention is superior in thehigher temperature range relevant to rolling resistance and also in thelow temperature range relevant to wet grip.

EXAMPLES 10 (COMPARATIVE EXAMPLE) AND 11

The following rubber blends (except for the sulfur and accelerator) wereprepared at 140°-150° C. (ejection temperature) in a 1.5 l kneader.Mixing time: 5 minutes. The sulfur and accelerator were incorporated atthe end at approx. 50°-70° C. on a roller.

Comparative Constituent Example 10 Example 11 Buna VSL 5025-0 (Bayer AG)70 0 Rubber according to Example 1 0 70 Buna CB 25 30 30 Corax N 121carbon black 50 50 (Degussa AG) Aromatic mineral oil 5 5 Antilux 654ozone stabilizing wax 1 1 (Rheinchemie) Zinc oxide RS (Bayer) 3 3Stearic acid 2 2 Vulkanox 4020 antioxidant (Bayer AG) 1 1 Vulkanox HSantioxidant (Bayer AG) 1 1 Sulfur 1.7 1.7 Vulkacit CZ 1.4 1.4 Vulkacit D0.3 0.3

The rubber blends were then vulcanized for 10 minutes at 170° C. Thevulcanized products had the following properties:

Comparative Property of vulcanized product Example 10 Example 11 Tensilestrength (MPa)⁽¹⁾ 18.9 20.8 Elongation at break (%)⁽¹⁾ 358 353 Tensilestress at 100% elongation 3.1 3.2 (MPa)⁽¹⁾ Tensile stress at 300%elongation (%)⁽¹⁾ 15.0 16.9 Shore A hardness (23° C.)⁽²⁾ 73 71 Shore Ahardness (70° C.)⁽²⁾ 66 65 Rebound resilience at 23° C. (%)⁽³⁾ 34 27Rebound resilience at 70° C. (%)⁽³⁾ 50 50 Difference between reboundresiliences 16 23 at 23° and 70° C. Tear resistance N/mm 14.9 23

The test results of Examples 10 and 11 clearly show that the favorableeffects according to the invention in terms of the damping behavior arenot restricted to silica-filled rubber blends but, surprisingly, arealso to be found in black-filled rubber blends.

EXAMPLE 12 Preparation of a Rubber Blend of Silica and a Solution of aRubber Containing 1 wt. % of Hydroxyl Groups

25 g of mercaptoethanol and 1 g of azobiscyclohexanenitrile were addedto a solution of 500 g of Buna VSL 5025-0 (solution styrene/butadienerubber with a styrene content of 25 wt. % and a 1,2-vinyl content of 50wt. %, from Bayer AG) and the mixture was heated for 16 hours at 80° C.2.5 g of Vulkanox BKF (phenolic antioxidant from Bayer AG), 420 g ofVulkasil S (highly active precipitated silica with a BET specificsurface area of 160-200 m²/g, from Bayer AG) and 196.9 g of Renopal 450(aromatic mineral oil from Fuchs Mineralölwerke) were then stirred in at70° C. and the solvent was distilled off by introducing steam. This gavea silica/rubber blend in which the silica was homogeneously distributed.The effluent was clear and free of silica. The moist silica/rubber blendwas dried at 70° C. under vacuum. The yield of dried rubber blend was1107 g (97% of theory).

EXAMPLE 13 (COMPARATIVE EXAMPLE) Preparation of a Rubber Blend of Silicaand a Solution of a Rubber Without Hydroxyl Groups.

500 g of Buna VSL 5025-0 (solution styrene/butadiene rubber with astyrene content of 25 wt. % and a 1,2-vinyl content of 50 wt. %, fromBayer AG) and 2.5 g of Vulkanox BKF (phenolic antioxidant from Bayer AG)are dissolved in 4 l of cyclohexane. 500 g of Vulkasil S (highly activeprecipitated silica with a BET specific surface area of 160 to 200 m²/g,from Bayer AG) are then added, the mixture is subsequently homogenizedby stirring for 45 minutes at 70° C. and the solvent is then driven offwith steam at 100-110° C. This gave a residue of rubber containinglittle silica. The bulk of the silica had collected in the effluent.

EXAMPLE 14 (COMPARATIVE EXAMPLE) Preparation of a Rubber Blend of Silicaand a Solution of a Rubber Containing 0.07 wt. % of Hydroxyl Groups

1.55 g of mercaptoethanol and 0.5 g of azobiscyclohexanenitrile areadded to a solution of 500 g of Buna VSL 5025-0 (solutionstyrene/butadiene rubber with a styrene content of 25 wt. % and a1,2-vinyl content of 50 wt. %, from Bayer AG) and the mixture is heatedfor 16 hours at 80° C. 2.5 g of Vulkanox BKF (phenolic antioxidant fromBayer AG), 401.2 g of Vulkasil S (highly active precipitated silica witha BET specific surface area of 160-200 m²/g, from Bayer AG) and 188.1 gof Renopal 450 (aromatic mineral oil from Fuchs Mineralölwerke) werethen stirred in at 70° C. and the solvent was distilled off byintroducing steam. This gave a rubber-containing residue containinglittle silica. The bulk of the silica had collected in the effluent andwas separated off by sieving. The rubber-containing residue was dried at70° C. under vacuum to give 821 g (75% of theory).

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A process for the preparation of rubber blendscontaining one or more solution-polymerized rubber comprising hydroxylgroups, synthesized from diolefins and vinylaromatic monomers, whereinsaid at least one solution-polymerized rubber containing primaryhydroxyl groups contain in the range of 0.1 to 5 wt. % of bondedhydroxyl groups, comprising the steps of a) adding to a solution of,said at least one solution-polymerized rubber containing hydroxylgroups, one or more fillers, in amounts ranging from 0.5 to 500 parts byweight, based on 100 parts by weight of rubber, and optionally otherworking-up and/or processing and/or stabilizing aids, and b) removingthe solvent.
 2. A process according to claim 1, wherein said solvent isremoved with the aid of steam.
 3. A molded part comprising rubber blendscontaining at least one solution-polymerized rubber comprising hydroxylgroups, synthesized from diolefins and vinylaromatic monomers, whereinsaid at least one solution-polymerized rubber containing primaryhydroxyl groups contain in the range of 0.1 to 5 wt. % of bondedhydroxyl groups.
 4. A tire comprising rubber blends containing at leastone solution-polymerized rubber comprising hydroxyl groups, synthesizedfrom diolefins and vinylaromatic monomers, characterized in that said atleast one solution-polymerized rubber containing primary hydroxyl groupscontain in the range of 0.1 to 5 wt. % of bonded hydroxyl groups.
 5. Atire tread comprising rubber blends containing at least onesolution-polymerized rubber comprising hydroxyl groups, synthesized fromdiolefins and vinylaromatic monomers, characterized in that said atleast one solution-polymerized rubber(s) containing primary hydroxylgroups contain in the range of 0.1 to 5 wt. % of bonded hydroxyl groups.